Pairing interaction from Demons in Sr$_2$RuO$_4$

TL;DR

This paper explores the role of a recently observed low-energy 'demon' mode in Sr$_2$RuO$_4$, showing it reduces Coulomb repulsion but does not induce overall attraction for electron pairing.

Contribution

It provides the first explicit calculation of the dynamically screened Coulomb interaction considering the demon mode in Sr$_2$RuO$_4$, highlighting its impact on pairing interactions.

Findings

Demon mode reduces Coulomb repulsion at relevant energies.

No total attractive pairing interaction from demon mode.

Regions in momentum-energy space where Coulomb interaction becomes attractive.

Abstract

We investigate the properties of the recently observed "demon" mode, a 3D acoustic plasmon, in Sr$_2$RuO$_4$ with an emphasis on evaluating its role for the pairing interactions in this superconductor. The demon mode is a low-energy electronic excitation, and it has been suggested that it could contribute to a reduced Coulomb repulsion and even a possible attractive interaction between electrons. In this study, we explicitly calculate the dynamically screened Coulomb interaction for Sr$_2$RuO$_4$ by using a renormalized tight-binding band structure and the random phase approximation for the dielectric function. Although the focus here is on Sr$_2$RuO$_4$, this material is considered mainly as a prototype system, having an observed demon mode, and our results should be considered as a guide for application to other systems. Our calculations show that there are regions in ($\mathbf{q}$, $\omega$) space where the Coulomb interaction becomes attractive. We find that, although the demon mode is not capable of producing a total attractive electron pairing interaction in Sr$_2$RuO$_4$, it does contribute to a significant reduction in the Coulomb repulsion at the relevant pairing energy scale.